This invention relates to ion sources used in Ion Assisted Deposition (IAD) of films, in particular optical quality films, and to methods of operating such ion sources.
Ion sources had their origins in space propulsion but more recently have found use in industrial processes such as IAD of thin film coatings. In an IAD process, an ion beam from an ion source is directed toward a target substrate to cause densification of the coating material as it is deposited. The process occurs within an evacuated chamber of pressure of the order 10xe2x88x922 Pa or less.
The benefits that result from ion assistance, during growth, of almost any optical material is well understood and is today widely practiced. In general, ion bombardment provides close to bulk density of the film resulting in dramatic improvements in durability and performance. However, for many classes of materials this benefit is accompanied by an undesirable modification of optical properties observed as an increasing absorption coefficient (k) and variability in refractive index (n). For many classes of materials, this problem results from incompatibility between the ion species and depositing material.
Argon and oxygen are the two most predominant species of ions used in IAD processes. The high momentum of Ar+ provides high packing density, although usually leads to a reduction of metal oxides and fluorine depletion of most metal fluorides. This results in metal-rich films with a subsequent increase in optical absorption.
The use of O+ is well suited to the IAD of metal oxides such as titania, silica etc. With the correct choice of energy and ion current density, O+ IAD can provide fully densified and low-stress films. Problems arise however where the very chemically active oxygen ions displace fluorine atoms from depositing molecules immediately prior to their incorporation in the film. This leads to the growth of oxy-fluorides with subsequent deterioration of optical properties. The extent to which this occurs depends on factors such as ion energy and current.
In a first aspect, the invention resides in an ion source including:
an ionization region;
a gas supply;
a gas excitation system;
ion influencing means; and
an ion source controller;
wherein said gas supply supplies an ionizable gas to said ionization region;
wherein said gas excitation system causes ionization of gas in said ionization region;
wherein said ion influencing means forms ions produced in said ionization region into an ion current substantially directed at a target;
and wherein said ion source controller controls said ion source so as to intermittently produce said ion current.
In a first embodiment, gas is intermittently introduced into the ionization region.
In a second embodiment, the flow of electrons into the ionization region is made intermittent.
In a further embodiment the ion source of the present invention is combined with a film deposition apparatus, the combined apparatus including a deposition control system that prevents deposition of new material onto the target substrate while the ion current is directed towards the target.